Security forces detained an unknown number of protesters and also several journalists, who were later released. But police allowed dozens of Sisi supporters to gather in restricted squares and areas.

Here's what's going on:

Why are people unhappy?

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Many of the protesters do not want to see the two Red Sea islands, Tiran and Sanafir, turned over to the Saudis. Egyptians have also been troubled by other aspects of Sisi's presidency including the struggling economy and the detention of political dissidents.

What's so important about these islands?

Earlier this month, thousands of protesters poured into the streets after Cairo first announced its intention to transfer the two islands, according to state-run news agency Al-Ahram.

The government's decision to cede the islands to the Saudis came after the kingdom announced investment projects in Egypt worth billions of dollars. Both countries' officials say that the islands always belonged to the Saudis, but were only temporarily under Egyptian control because of a request from Riyadh in 1950.

This explanation has not alleviated anger in Egypt and has led to accusations that the government is selling the islands to its neighbor.

The islands, which sit between the two countries, have a strategic importance as ships pass by on their way to Jordan and Israel. Critics viewed Sisi's decision as a betrayal of Egypt's former president, Gamal Abdel Nasser, who had viewed the islands as critically important.

The island deal still requires approval by the parliament, according to Al-Ahram.

Is the date significant?

Activists planned the protest on Sinai Liberation Day, which marks the day when Israeli forces withdrew from the Sinai Peninsula in 1982.

Sisi slammed the protest organizers, describing them as people who seek to disturb Egypt's security, Al-Ahram reported Sunday.

"We have exerted a lot of effort to achieve security and stability," he said in televised remarks. "There are people who look to shake this stability."

Egypt's army posted pictures on its official Facebook page of its military vehicles deployed on public streets as a show of force.

Egypt's interior ministry had warned that it would not tolerate attempts to "undermine the country's security," according to Egypt's state media.

Activists spread word of the protest on social media using the hashtag #April25.

What's the current political climate in Egypt?

Dissatisfaction in Egypt extends beyond the issue of the islands to questions over a stalled economy, government crackdowns on civil society, restrictions on NGOs and the detention of political rivals under Sisi's government. Egypt also faces security issues in the Sinai Peninsula, where militants have formed a local ISIS affiliate.

The Muslim Brotherhood, which opposes Sisi, has called for "no compromise on an inch of Egyptian land."